1. Field of the Invention
This invention relates to a thin-film semiconductor element such as a thin film transistor, a thin film diode, etc. for use in a switching element, an integrated circuit, a liquid crystal display element, and so on.
2. Description of Prior Art
A thin film semiconductor element such as a thin film transistor, a thin film diode or the like has been researched and developed as an element which can be formed on an insulating substrate, for example, in a driving circuit for a liquid crystal display device or an amplifier for an image sensor. Likewise, in a monolithic technique in which circuit elements are formed on a semiconductor element or semiconductor substrate, the thin film transistor has been also tentatively used to improve integration of the circuit elements and obtain a cubic structure of the circuit. As a thin-film semiconductor material for the above technique has been adopted polycrystalline material such as Polysilicon, amorphous material such as amorphous silicon, or semi-amorphous semiconductor such as semi-amorphous silicon which is intermediate in material property between the polycrystalline material and the amorphous material and has both of polycrystalline and amorphous properties.
However, the carrier mobility of the thin film semiconductor as described above is remarkably small, for example, one-several to one-several tens of that of monocrystal material, and thus the operating speed of a semiconductor element using the semiconductor materials as described above is also remarkably low. For example, the amorphous silicon has an electron mobility below 1 cm2/Vs, and a general polysilicon has an electron mobility of 10 to 30 cm2/Vs. Even if a special method such as a laser annealing method is adopted, the electron mobility is limited to 200 cm2/Vs at the maximum, and this value is remarkably small in comparison with 1350 cm2/Vs, the electron mobility of monocrystal silicon. Therefore, the thin film semiconductor element has been mainly used in a relatively-low frequency field or as an auxiliary element for the monocrystal semiconductor such as a load resistance element of a static RAM.
It would be considered as a cause of the low carrier mobility of the thin film semiconductor that a carrier scattering is liable to occur in the amorphous material because the amorphous material has a short crystal periodicity, and the mean free path of carriers becomes shorter. On the other hand, it would be considered for the polycrystalline material that foreign elements are concentrated at grain boundaries and a barrier is liable to occur at the grain boundaries, so that the carriers are randomly scattered at the grain boundaries. On the basis of the above consideration has been made an attempt that each crystal is designed to be larger in size to reduce the number of grain boundaries per unit length, whereby the mobility is increased. The semi-amorphous material mainly comprises a portion having long periodicity as a whole, like the polycrystalline material, and has no distinct grain boundaries, so that the carrier scattering at the grain boundaries is depressed, and a relatively-high mobility is obtained. However, it is difficult to obtain semi-amorphous material having large grain diameter (an area in which an orderly state is kept over a long distance). In addition, polysilicon having large grain diameter is easily obtained, but characteristics of an element is greatly scattered because the size of the element and the grain diameter are in the same dimension. Such an element can not be practically used.
An object of this invention is to provide a semiconductor material which provides an improved carrier mobility in a thin film semiconductor, and is suitable for a thin film semiconductor element.
The inventors of this application have studied the cause of the increase of the carrier mobility by a laser annealing method. For example, irrespective of the same crystal size of several to ten xcexcm order, the electron mobility of 200 cm2/Vs is obtained by a laser annealing process while the electron mobility of 30 cm2/Vs is obtained in a heat annealing process using an electric furnace. One of causes of introducing the difference in the electron mobility is guessed to reside in that the heat annealing process requires a long-time annealing to cause the concentration of foreign elements at a grain boundary, while the laser annealing process, particularly the laser annealing process using a pulse laser requires a short-time annealing which is insufficient for the foreign elements to concentrate at the grain boundary, and the barrier formation at the grain boundary is incomplete in the laser annealing process.
In addition to the above cause, it is guessed that the laser annealing process causes a stress occurring in a laser irradiation to be conserved in the material, and any affection is given to the grain boundary by this conserved stress. That is, close junction is made between the grain boundaries by the laser annealing, and the width of a barrier at the grain boundary is shortened.
In order to verify the above guesses, an experiment as shown in FIG. 1 was made. That is, a coating of insulating material which is contracted by heat (having a thermal contractive property), such as phosphosilicate glass, borosilicate glass, phosphoborosilicate glass, AN glass or quartz glass, was formed on a semiconductor substrate 101 by a plasma CVD method or a sputtering method, and then a coating of thin-film semiconductor material such as polysilicon, amorphous silicon or semi-amorphous silicon was further formed on the coating of the contractive insulating coating by the plasma CVD method or the sputtering method. An attention should be given to the difference between the terms of thermal contraction and thermal expansion. The latter has reversibility in a heat cycle, while the former has no reversibility in the heat cycle, that is, has irreversibility in the heat cycle. Therefore, the laminating process using the above materials must be carried out at such a sufficiently low temperature that the thermally-contractive insulating coating is not contracted.
The semiconductor coating and the insulating coating were subjected to a patterning process to form an intermediate as shown in FIG. 1(B). Thereafter, the intermediate was heated at a proper temperature to contract the insulating coating and thus the semiconductor coating on the insulating coating. The contracting temperature is dependent on insulating material of a substrate, and is optimumly about 600xc2x0 C. for the substrate (coating) of quartz, for example. The amorphous silicon coating is crystallized at 600xc2x0 C. The intermediate was kept at this state for 24 hours.
Thereafter, in order to neutralize or compensate the disorder of crystal due to the above annealing process, the intermediate was subjected to a heat-annealing at 200 to 400xc2x0 C. in hydrogen atmosphere to add hydrogen to polysilicon which has been formed through the annealing process. Elements having an MOS-structure was formed using the thin-film semiconductor obtained in the above process to measure the electron mobility thereof, and the electron mobility of 40 to 60 cm2/Vs was obtained as a measured result. In comparison with the electron mobility of the thin-film semiconductor formed on a substrate material having non-contractive property, this mobility was larger by 30 to 100%.
This experimental result is not a direct proof that the laser annealing process provides high mobility, however, it has been incidentally recognized through this experiment that using a special substrate material, the semiconductor coating on the substrate can be supplied with stress, and the mobility thereof can be improved. The inventors of this application have further proceeded with their study on the basis of this recognition, and has implemented this invention.